This invention relates to glass compositions for use in encapsulating electronic devices and, more particularly, to glass compositions having controlled expansion properties which can be used in glass compression seals for insulating the electrical leads of semiconductor device packages.
Insulating glasses which melt at temperatures exceeding 500.degree. C. are conveniently used to provide insulated electrical feedthroughs in the walls of many semiconductor device enclosures. The glass serves to insulate the electrical lead from the body of the device enclosure, and at the same time to prevent the ingress of atmospheric contaminants into the interior of the enclosure where they might adversely affect the reliability of the semiconductor device contained therein. Leads sealed so as to be impervious to atmospheric contaminants are said to be "hermetic."
Compression glass seals are a well-known technique for providing hermetic insulating feedthroughs in electrical enclosures, particularly semiconductor device enclosures. In order to insure that all parts of the seal remain in compression, the coefficient of expansion of the lead is generally chosen to be less than the coefficient of expansion of the glass surrounding the lead, which is in turn chosen to have a coefficient of expansion substantially less than the coefficient of expansion of the portion of the encapsulation body which surrounds the glass. In a typical example of the prior art, a compression glass seal is made using 52-Alloy for the central lead, surrounded by Corning #9013, 9048, or 9155 sealing glass which is inset in a mild (1010-Alloy) steel base. Hermetic compression glass seals are obtained using these combinations of materials. SHOTT 8421 glass manufactured by the Jenaer Glaswerk is another example of a prior art sealing glass suitable for use with 52-Alloy leads and 1010-Alloy bases. 52-Alloy and 1010-Alloy are American National Standard designations defined in ANSI/ASTM Standard Specification F30-77 and A29-79 respectively. These materials are well known per se in the art.
It has been found, however, that compression glass seals made according to the prior art are not sufficiently rugged for effective use in semiconductor devices which may be subjected to rough handling, a condition often encountered during the latter stages of manufacturing. For example, TO-3 type semiconductor packages have a mild steel base in which are inset two or more compression glass seals for the two insulated leads to be connected to the semiconductor die placed on the base of the package. During assembly, these leads frequently become bent and must be straightened one or more times before manufacturing is completed. It has been observed that the hermeticity of the seal between the electrical leads and the glass is seriously degraded by bending and straightening, so that the probability of having a non-hermetic device package increases significantly depending upon the number of times that the leads have been bent and straightened. This property reduces both the manufacturing yield and the field reliability. Improper selection of the relative coefficient of expansion of the different parts of the seal, particularly the sealing glass, is believed to be a significant cause of the tendency to lose heremticity following one or more lead bending and straightening operations.
Accordingly it is an object of the present invention to provide an improved glass composition for the fabrication of compression glass seals and electrical devices.
It is a further object of this invention to provide a sealing glass composition with improved thermal expansion properties.
Another object of this invention is to provide an improved method for making hermetic seals for electrical devices.
It is an additional object of this invention to provide a semiconductor device encapsulation of improved properties, particularly the ability to withstand rough handling and multiple lead bending-straightening operations without loss of hermeticity.